Treatment of biological tissues with glutaraldehyde is a preferred method for manufacturing bioprosthetic implants. Glutaraldehyde forms cross-links and polymers with a variety of matrix components of biological tissues, rendering these tissues less immunogenic when implanted in a living host. Glutaraldehyde cross-linking also alters the mechanical properties of biological tissues.
Glutaraldehyde cross-linked porcine heart valves and bovine pericardial tissues have been used as material for bioprosthetic human heart valves. Glutaraldehyde treated blood vessels and pericardial patches have also been used in other applications.
Glutaraldehyde treated biological tissues, especially porcine heart valves and bovine pericardial tissues, have been used as materials for bioprosthetic heart valves for more than 20 years. It has become evident that calcification of the glutaraldehyde treated bioprosthetic heart valves has been a major cause of long term device failure.
The exact mechanism underlying the calcification of glutaraldehyde treated heart valves is not known. It has been suggested that the toxicity of the glutaraldehyde polymers or the breakdown products of the glutaraldehyde polymers cause the death of connective tissue cells arriving at the implant site. The remnants of these dead cells may then act as nidi for the nitiation of calcificaiton. The toxicity of the glutaraldehyde polymers which leach out to tissue surrounding a glutaraldehyde cross-linked bio-implant may, through a similar mechanism, further complicate the integration of the implant into the host tissue. The free aldehyde groups of the glutaraldehyde polymers may contribute to the toxicity of the polymers.
A variety of approaches have been taken to counteract calcification of glutaraldehyde treated implants. For example, aldehyde treated tissues have been reacted with polyols (U.S. Pat. No. 5,476,516, issued to Seitter et al., 19 Dec. 1995), treated with alipathic carboxylic acids (U.S. Pat. No. 4,976,733 issued to Girardot 11 Dec, 1990) treated with partially degraded heparin (U.S. Pat. No. 5,645,587 issued to Chanda et al. 8 Jul. 1997), treated with sodium dodecyl sulfate (U.S. Pat. No. 4,323,358 issued to Lentz et al. 6 Apr. 1982), treated with polysaccharides, disphsphonates or phosphoproteins (U.S. Pat. No. 5,104,405 to Nimni 14 Apr. 1992), impregnated with ferric or stannic salts (U.S. Pat. No. 5,002,566 to Carpentier et al. 26 Mar. 1991), and treated with trivalent aluminum cations (U.S. Pat. No. 5,094,661 to Levy et al. 10 Mar. 1992).
Sodium borohydride in aqueous solution is a known reducing agent. Sodium borohydride in solution has previously been used as a reducing agent to treat glutaraldehyde fixed tissues. U.S. Pat. No. 4, 553,974 issued to Dewanjee on 19 Nov. 1985 discloses a process for treatment of glutaraldehyde fixed tissue with a calcification inhibiting agent, and then reducing the tissue with sodium borohydride. The calcification inhibiting agent contains reactive amino groups that bond to free reactive groups in the fixed tissue. Dewanjee '974 discloses a variety of amino diphosphonate compounds as effective calcification inhibiting agents (as do U.S. Pat. Nos. 5,296,583 and 5,436,291 issued to Levy et al. on 22 Mar. 1994 and 25 Jul. 1995 respectively). Dewanjee '974 teaches that a solution of sodium borohydride may be used to stabilize the bonding of such amino diphosphonates and glutaraldehyde to protein molecules.
Chen et al. ("Effect of 2-amino oleic acid exposure conditions on the inhibition of calcification of glutaraldehyde cross-linked porcine aortic valves" Journal of Biomedical Materials Research, Vol. 28, 1485-1495 (1994)) teach that the use of a sodium borohydride solution alone to treat glutaraldehyde fixed aortic tissues is not effective to reduce calcification in the rat subdermal implant model. In fact, when used on aortic wall tissue, the sodium borohydride treatment resulted in increased calcification compared to controls with no treatment.